This saying is originally from a rather obscure meme that includes Max and Ruby. At the end of the comic, Max exclaims "BEEP BEEP LETTUCE" in response to a question Ruby asks.
Essentially, Beep beep lettuce can be used as an interjection whenever one feels it's appropriate or just want to mess around with whoever they are with.
Person 1: Just grab me the Ragu from the stock room, ok Jimmy?
Jimmy: BEEP BEEP LETTUCE
Person 1: ...
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